As mentioned in the previous paragraph and as the disclosure continues, the reader will recognize (and will occasionally be reminded) that the present invention is useful for many purposes and in many environments. However, for narrative convenience, the invention will be described as functioning to assist in the identification of radar targets by the analysis of the frequency spectrum of reflected radar signals, particularly when processed as disclosed in the related patent application, Ser. No. 05/001,949.
Radar has been intensively developed because it has the ability to detect the presence, velocity, acceleration, range, height, azimuthal direction and direction of movement of a target. The exact nature of the target is, however, usually unknown.
Automatic interrogation and IFF equipment and procedures have been developed that provide an identification of civil airliners and friendly military aircraft. This approach to identifying a radar target is inherently less than completely satisfactory because enemy targets are not identified and because of the possibility of transponder malfunction and of the possibility of the enemy developing the ability to deceptively respond to IFF interrogation.
Because of the very obvious desirability of being able to determine the nature of the radar target from the radar return (ie. the signal caused in the radar receiver by the target) great efforts have been, and are being, made to identify targets from the characteristics of the reflected signal. These reflected signal characteristics are often called the target signature.
In identifying targets by their radar signatures, difficulty is often encountered because of the xe2x80x9csmearingxe2x80x9d of the spectral signature as a result of component frequency shifting. This shifting is caused by many factors, two of which are particularly troublesome and which can occur because of deliberate radar countermeasures initiated by the enemy pilot.
One radar countermeasure which the enemy pilot may use is to change the radial velocity of his aircraft relative to the radar by maneuvers such as weaving, changing speed, etc. These maneuvers will produce a shifting of the carrier frequency and a frequency translation of much of the spectral signature of the target. The invention disclosed in the related patent application Ser. No. 05/001,949 functions to negate the effectiveness of such a countermeasure by folding the spectral signature about the carrier frequency which is translated to zero frequency.
A second radar countermeasure, which the enemy pilot may use, is to vary the engine speed of his aircraft. Because of the inherent time lag, the change of engine speed and the change of airframe velocity can be considered as being almost independent countermeasures. The effect of changing engine speed is to vary the frequency of harmonically related spectral components which result from the radar reflections from structures, such as engine accessories, compressor rotor discs, propellers, etc. which operate at harmonically related speeds. When, as in the invention of the related patent application, Ser. No. 05/001,949, the effect of varying radial airframe velocity is removed and the target signature is displayed with component lines folded about the carrier frequency translated to DC, the use of the changing engine speed countermeasure results in a smearing of the harmonically related spectral lines.
This type of smearing, ie. where the nth harmonic frequency is shifted n times the fundamental component frequency shift, is soften described in terms of a variation of the spectral extent and in terms of frequency dilation and contraction. Both terms will be used herein.
It should also be mentioned that the same problem of harmonic smearing with variations of the spectral extent is encountered in many simpler environments, such as in a spectral display of a varying frequency sinusoidal signal that includes harmonics.
In efforts to avoid spectral smearing and variations of the spectral extent, prior investigators have resorted to studying very short time samples of the spectral display. This method, while minimizing the effect of frequency variations, has the inherent disadvantage of limiting the spectral resolution and of discarding much information.
This invention seeks to avoid the disadvantages of prior methods of spectral analysis, particularly in connection with radar signature analysis where smearing of the spectrum display occurs as the result of frequency dilation and contraction, by providing circuitry which stabilizes the spectral extent.
More specifically, the invention provides means which store the radar return signal at a fixed rate and which varies the storage readout rate in such a way that one of the harmonically related components, usually the fundamental frequency component, is kept constant. This is accomplished by controlling the readout speed with a variable frequency clock that is controlled by the output of a discriminator circuit which receives the selected frequency component through a band pass filter. Converters are used to allow digital storage while having input and output signals in analog form.
It is, therefore, an object of the invention to provide improved means for signal analysis.
Another object is to provide a circuit for stabilizing the spectral extent of a signal having harmonically related components by storing the signal at a fixed rate and by varying the time compression or read out rate in such a manner that the frequency of one of the components of the read out signal is kept constant.
Yet another object is to provide a circuit for stabilizing the spectral extent of a signal having harmonially related components by storing the signal at a fixed rate and controlling the time compression or read out rate by a discriminator circuit which is connected to receive a selected frequency component of the read out signal.
A still further object of the invention is to provide an improved circuit for analysing radar return signals having harmonically related components wherein the return signals are stored at a fixed rate in digital form and are read out and converted to analog form at a varying rate that is conrtrolled by a discriminator circuit which is connected to receive a selected one of the harmonically related components of the analog read out signal.